A thermal radiation or infrared (IR) thermometer is a device capable of measuring temperature without physically contacting the object of measurement. Thus, such thermometers are often called “non-contact” or “remote” thermometers. In an IR thermometer, the temperature of an object is taken by detecting an intensity of the IR radiation which is naturally emanated from the object's surface. For objects between 0 and 100° C., this requires the use of IR sensors for detecting radiation having wavelengths from approximately 3 to 40 micrometers. Typically, IR radiation in this range is referred to as thermal radiation.
One example of an IR thermometer is an “instant ear” medical thermometer, which is capable of making non-contact temperature measurements of the tympanic membrane and surrounding tissues of the ear canal of a human or animal. Instant ear thermometers are exemplified by U.S. Pat. No. 4,797,840 to Fraden (“the '840 Patent”), which is incorporated by reference herein in its entirety. Other examples include medical thermometers for measuring surface skin temperatures (for example, a skin surface temperature of the forehead) as exemplified by U.S. Pat. No. 6,789,936 to Kraus et al., which is incorporated by reference herein in its entirety.
In order to measure the surface temperature of an object by its IR radiation emissions, the IR radiation is detected and converted into electrical signal suitable for processing by conventional electronic circuits. The task of detecting the IR radiation is accomplished by an IR sensor or detector.
Conventional thermal IR sensors typically include a housing with an infrared transparent window and at least one sensing element which is responsive to a thermal radiation energy flux Φ emanating from the object's surface to pass through the IR window of the IR sensor. The IR sensor functions to generate an electric signal which is representative of the net IR flux Φ existing between the sensing element and the object of measurement. The electrical signal can be related to the object's temperature by appropriate data processing as is for example further described below.
Thermal flux Φ is a function of two temperatures: a sensing element surface temperature Ts and a surface temperature of the object Tb (measured in degrees Kelvin). Theoretically, thermal radiation is known to be governed by Planck's law. However, for a broad optical spectral range, which may be determined by an optical system of the IR thermometer, the relationship between the two temperatures Ts, Tb and the flux Φ may be approximated by a fourth-order parabola. In physics, this approximation is known as the Stefan-Boltzmann law:Φ=κεbκsσ(Tb4−Ts4)  (1)
where εb and εs are the surface emissivities of the object and sensing element, respectively, σ is the Stefan-Boltzmann constant, and k is an optical constant which may be determined by measurement during calibration of the IR thermometer.
For a relatively small difference between the true object's temperature Tb and sensor's temperature Ts Eq. (1) can be simplified as:Φ≈4κεbεsσTs3(Tb−Ts)  (2)
An ultimate purpose of an IR thermometer is to determine the surface temperature of the object (Tb), which may be calculated as Tbc from inverted Eq. 2:
                              T          bc                =                              T            s                    +                      Φ                          4              ⁢              κ              ⁢                                                          ⁢                              ɛ                b                            ⁢                              ɛ                s                            ⁢              σ              ⁢                                                          ⁢                              T                s                3                                                                        (        3        )            
Ideally, the computed temperature Tbc should be equal to the true temperature Tb. Practically, these temperatures may differ as the result of error. It can seen from Equation (3) that, in order to calculate temperature Tbc, two values need to be determined: the magnitude of the IR flux Φ and the IR sensing element's surface temperature Ts. The accuracy of the temperature computation depends on the measurement accuracy for all variables at the right side of Eq. (3). The first summand Ts can be measured quite accurately by a number of techniques known in the art, for example, by employing a thermistor or RTD temperature sensor. The second summand can be more problematic, especially due to a generally unknown and unpredictable value of the object's emissivity εb,. For example, in medical thermometry, the emissivity εb. is a skin emissivity that is defined by the skin properties and shape. The skin emissivity may, for example, range from 0.93 to 0.99. To determine how emissivity affects accuracy, a partial derivative of Eq. (2) may be calculated as:
                                          ∂            Φ                                ∂                          ɛ              b                                      =                  4          ⁢                      κɛ            s                    ⁢          σ          ⁢                                          ⁢                                    T              s              3                        ⁡                          (                                                T                  b                                -                                  T                  s                                            )                                                          (        4        )            
The partial derivative represents the measurement error due to an unknown emissivity εb of an object. Eq. (4) shows that the error essentially approaches zero when temperature Ts of the sensor approaches temperature Tb; of the object, that is when Tb≈Ts. Thus, to minimize errors, it is desirable to keep the temperature Ts of the IR sensor as close as is practical to the object's temperature Tb. For an instant ear thermometer, for example, U.S. Pat. No. 5,645,349 to Fraden teaches a heated sensing element for bringing the temperatures Ts, Tb into proximity U.S. Pat. No. 7,014,358 issued to Kraus et al. alternatively teaches a heating element for warming the IR sensor housing. U.S. Pat. No. 5,645,349 and U.S. Pat. No. 7,014,358 are each incorporated by reference in its entirety herein.
When temperature is measured from a surface, it is important to direct the associated IR radiation flux Φ to the IR sensor only from the measured surface, and not from any stray objects that may appear in the field of view of the optical system. IR radiation from stray objects alters the measured flux, and thereby contributes to error.
One way to minimize the chance of picking up flux from stray objects is to narrow the optical field of view of the IR thermometer. One method of using IR lenses to narrow the optical field of view is exemplified by U.S. Pat. No. 5,172,978 to Nomura et al. (radiant thermometer including a lens barrel mounting a condensing lens at one end and an IR detector at the other end) and U.S. Pat. No. 5,655,838 to Ridley et al. (radiation thermometer with multi-element focusing lens, eye piece, beam splitter and IR detector), each of which is incorporated by reference in its entirety herein.
Another method for minimizing the chance of picking up flux from stray objects employs curved mirrors to narrow the field of view. This approach is exemplified by U.S. Pat. No. 4,494,881 to Everest, which is incorporated by reference in its entirety herein.
These methods successfully solve the problem of eliminating stray IR signals from surrounding objects, but remain ineffective in further preventing stray radiation from interior components of the IR thermometer that surround the IR sensor. This source of stray radiation is unaffected by efforts to limit the optical field of view. It would be of significant benefit to develop an IR thermometer having an IR sensor that is unaffected by stray radiation from interior components of the IR thermometer that surround the IR sensor.